This invention relates in general to audio turn-on circuits and in particular to such circuits used in connection with integrated circuit audio amplifiers in television receivers.
The trend in television (TV) receiver audio systems is toward greater power and higher sound quality. There is also a strong movement to use integrated circuits where possible in TV receivers to obtain the benefits of standardization, reliability, compactness and reduced cost. In addition to an output terminal, most integrated circuit amplifiers include a so-called inverting terminal and a noninverting terminal. These terminals correspond generally to a signal input (noninverting) terminal and a feedback input (inverting) terminal, with the output terminal being connected by a feedback circuit to the inverting terminal.
Difficulties arise in the environment of a TV receiver, where there is a great deal of extraneous signal energy present that needs to be filtered and decoupled from the audio system. Upon turn-on of the TV receiver, the audio amplifier becomes operational too rapidly and amplifies any signals at its input which may include a great deal of noise, since the TV receiver reaches stable operating conditions much more slowly. The audio circuit may also come on with a very noticeable "pop" or click despite the absence of noise at its input, because of rapid transitions in output potential resulting from fluctuation of the potentials at its inverting terminal and its noninverting terminal. Either one of these conditions can be objectionable to a listener.
There are a number of solutions to the problem, the obvious one of which is to use large filter capacitors across the input terminals of the amplifier to force a very slow turn-on and to assure that the two input terminals do not experience abrupt potential changes with respect to each other. Unfortunately, slow turn on is not an acceptable solution since the user expects the audio system to come on within a reasonably short time after turn-on of the TV receiver. Other circuits that do not require unacceptably long turn-on times have been used. In general they are quite elaborate and involve a number of transistors or diodes, in addition to resistors and capacitors for essentially holding the amplifier cut-off during turn-on of the TV receiver and gradually enabling it to reach operating potential. These circuits are costly and difficult to control in large scale manufacture and still impose substantial delays in audio turn-on. In some circuits the operating potentials and charging time constant for the filter coupled to the input terminal of the amplifier have been adjusted to the point where the sound quality has been compromised. There is thus a need in the art for a low cost audio turn-on circuit that avoids these problems.